Cofilin (CF) is an actin regulatory protein that plays a critical role in actin filament dynamics in a variety of cells. Two cofilin isoforms. muscle-type (M-CF) and nonmuscle-type (NM-CF) encoded by different genes, exist in mammals; in the adult, the former is predominantly expressed in muscle tissues, while the latter is distributed in various non-muscle tissues (Ono et al., 1994). In this study, we examined cofilin isoform expression during skeletal and cardiac muscle development in mice using cDNA probes and antibodies which distinguish the isoforms. We found that the expression of M-CF was initiated in terminally differentiated myogenic cells in both the myotome and limb buds. In myogenic cell cultures, its expression occurred coupled with myotube formation. NM-CF was expressed in developing skeletal and cardiac muscles but disappeared from skeletal muscle during postnatal development, while its expression persisted in the heart, even in adult mice. A similar situation was observed in the heart of other mammals. Thus, it is likely that the both cofilin isoforms are involved in the regulation of actin assembly during myofibrillogenesis. Only M-CF could be involved in actin dynamics in mature skeletal muscle, while both isoforms could be in the mature heart.